This invention relates to a catalytic reactor for use in automotive engine exhaust lines and of the type having a tubular housing which confines therein a catalyst preferably carried on a monolithic core.
Catalytic converters or reactors are now in wide use in automotive engine exhaust lines with the purpose of converting carbon monoxide, hydrocarbons and/or nitrogen oxides contained in the engine exhaust gas into harmless substances. In many cases a catalytic reactor for this purpose utilizes a monolithic carrier or core which is treated with a catalyst and has a cylindrical shape in a broad sense and a structure adapted for a generally axial gas flow therethrough. A typical example is a core of honeycomb structure having a generally elliptical cross-sectional shape. A major portion of a reactor housing to confine therein such a catalytic core is made up of a pair of shells flanged sideways and joined together along a plane containing the longitudinal axis of the housing. This major portion of the reactor housing has a larger cross-sectional area than exhaust pipes and is almost entirely occupied by the catalytic core. The housing, therefore, makes this portion as its mid-portion and has a foreportion through which exhaust gas is introduced into the mid-portion and an aft-portion through which the treated exhaust gas is discharged. The fore- and aft-portions are required to have a sufficiently high rigidity since they must support the mass of the mid-portion including the core and obstruct the transmission of shocks and vibrations from other components of the exhaust system to the mid-portion. In many cases, the fore- and aft-portions each take the form of a relatively thick cylinder, with a coupling flange welded at one end thereof for coupling with exhaust pipe, and are welded to the mid-portion.
A reactor housing of this construction involves a problem that the respective weld joints between the mid-portions, i.e. an assembly of two shells made from a relatively thin plate (e.g. 1.0-1.5 mm), and the fore- and aft-portions of a larger wall thickness (e.g. 2.0-2.5 mm) are rather weak to shocks and vibrations and tend to become the origin of cracks in the shells. This problem may be solved by increasing the wall thickness of the shells, but in practice such countermeasure is quite undesirable because of increasing difficulties in the press-forming of the individual shells and welding of the two shells, increasing weight of the housing and increasing cost of the material (the housing must be made of a heat- and corrosion resistant alloy).
Besides, it is difficult to weld the side flanges of the mid-portion at their fore- and aft-ends to the cylindrical fore- and aft-portions of the housing with high strength or rigidity of the resultant joints.
Regarding the interior of the reactor, it is necessary that the catalyst-treated core is held immovable and isolated from the inner surfaces of the shells since the core is usually made of a ceramic material and hence is fragile. A prevailing method of meet this necessity is the provision of a cushioning or buffer layer around the outer periphery of the core so as to be tightly interposed between the core and the inner surfaces of the shells in the assembled reactor. Usually wire mesh of stainless steel is employed as the material of the buffer layer by reason of its adequate resilience, flexibility, resistance to exhaust heat and suitability to constitute a buffer layer which does not allow a free passage of gas therethrough. Though the core in the assembled housing is clamped by the two shells because of the buffer layer being compacted upon coupling of the two shells, there is still a possibility that the core makes an axial movement during use of the reactor. Accordingly it is a usual way that the housing has in its interior ring-like retainers located at fore- and aft-boundaries of its mid-portion to hold the catalytic core securely in the axial direction. However, this makes the construction of the reactor very complicated and inevitably raises the cost of the reactor production.